1. Field of the Invention
The present invention is directed to a system which detects redundant images in a video sequence. In particular, the invention is directed to a system which determines whether a target image in the video sequence is a redundant image by comparing two predetermined threshold values to sums of differences between pixels in the target image and corresponding pixels in at least one other image in the video sequence.
2. Description of the Related Art
Telecines are well known devices that are used to generate video data from film images. These devices are commonly used in the television industry to convert motion pictures into video sequences for display on television. Problems arise from this conversion process, however, due to inherent differences between film and video.
More specifically, film sequences, such as motion pictures, have a frame rate of 24 frames-per-second. In contrast, video sequences have frame rates which are higher than 24 frames-per-second. For example, video sequences comprised of interlaced video images, i.e., video images in which two different fields are interlaced in one frame, have a frame rate of 30 frames-per-second. Video sequences comprised of progressive video images, on the other hand, have an even higher frame rate of 60 frames-per-second. In order to compensate for these different frame rates, and still maintain correspondence between the film sequence and the video sequence, telecine devices introduce additional, redundant images (i.e., fields and/or frames) into the video sequence. These redundant images essentially comprise repeats of earlier images of the video sequence, which are inserted at appropriate points in the video sequence.
Early on, it was recognized that there is no need to code these redundant images, since such images could merely be recreated at a decoder by copying other images of the video sequence. Accordingly, it became standard practice to include, in an encoder, circuitry for detecting redundant images. Conventional circuitry of this type determines redundancy by comparing pixels in an entire field and/or frame of a video sequence to corresponding pixels in another field and/or frame of a video sequence (see, e.g., U.S. Pat. Nos. 5,452,011, 5,406,333, 5,491,516, and 5,426,464). Specifically, conventional circuitry compares sums of differences in the foregoing pixels to a threshold value and, based on this comparison, ascertains whether the content of a target image is the same as the content of another image in the video sequence, usually an immediately preceding image.
To account for noise introduced by a telecine device, redundancy detection circuitry of the type described above sets its threshold high for high-noise telecine devices and low for low-noise telecine devices. However, using the same threshold to account for both variations in image content and telecine noise can lead to errors, particularly in cases where only small localized regions of two images vary in content. That is, in such cases, conventional circuitry often interprets these variations in content as telecine noise, thereby leading to an erroneous redundancy determination.
Thus, there exists a need for a redundant image detection circuit, for use in an encoding system or the like, which is able accurately to detect both telecine noise and variations in image content.